1. Field of the Invention
The present invention relates to an apparatus and method for reduction of nitrogen oxide emissions in flue gas using natural gas as a secondary reburn fuel when burning liquid fuels, such as coal water slurry or oil, or solid fuels, such as coal, as the primary fuel in the reburn combustion zone of a furnace.
2. Description of the Related Art
In the combustion of fuels with fixed nitrogen such as coal, oxygen from the air combines with the nitrogen to produce nitrogen oxides. At sufficiently high temperatures, oxygen reacts with atmospheric nitrogen to form nitrogen oxides. Nitrogen oxides are toxic and contribute to acid rain making the rain, dew and mist corrosive. Numerous government regulations limit the amount of nitrogen oxide which may be emitted from a combustion furnace and there is a need for apparatus and processes which reduce the nitrogen oxide emissions in furnace flue gas.
Numerous attempts have been made to develop apparatus and processes which reduce the nitrogen oxide emissions in a furnace flue gas. One such approach is a process known as in-furnace nitrogen oxide reduction, reburning, or fuel staging. In reburning pulverized coal, oil, gas, or other fuel is injected just downstream of a normal flame zone to form a fuel-rich reburning zone. Hence the nitrogen oxides are reduced to ammonia and cyanide-like fragments and N.sub.2. Subsequently, air is injected to complete the combustion process. The reduced ammonia and cyanide-like fragments then react to form N.sub.2 and nitrogen oxide.
Several problems are present with these prior art processes. First, coal is less efficient than natural gas as a reburn fuel because of its lower volatility and higher fixed-nitrogen composition. Within any furnace there are wide temperature zones in which fuel nitrogen will convert to nitrogen oxide. Thus, the fixed nitrogen reduced from the coal has a chance of ending up as nitrogen oxide.
Furthermore, the reburn fuel must be injected with a sufficient volume of air if air or flue gas containing oxygen is used as the carrier gas. There must be enough fuel to consume the oxygen in the carrier, and to supply an excess of fuel so reducing conditions exist. This increases the amount of fuel which must be used as reburn fuel. Furthermore, the necessity of using carrier air requires extensive duct work in the upper part of the furnace.
Additionally, the reburn fuel must be injected well above the primary combustion zone of the furnace so that it will not interfere with the reactions taking place therein. However, this fuel must be made to burn out completely without leaving a large amount of unburned carbon. To do this, the fuel must be injected in a very hot region of the furnace some distance from the furnace exit. The exit temperature of the furnace must be limited in order to preserve the heat exchanger surface. Therefore, a tall furnace is required to complete this second stage process.
Because of these mentioned problems with coal as a reburn fuel most of the reburn fuels used to date have been fluid fuels such as natural gas. The natural gas is injected into the reburn stage of the furnace in numerous ways.
In U.S. Pat. No. 4,779,545, a reburn process is disclosed wherein natural gas is introduced into the upper furnace through pulse combustors. The patent teaches that the natural gas must be injected in pulses to achieve NO.sub.x reduction. This process does not require any carrier air or flue gas for NO.sub.x reduction. However, it does require the expense of obtaining and operating pulse combustors and some air may be required. Therefore, there is a need for an improved process for in-furnace reduction of nitrogen oxides which can be implemented at low cost due to the fact that natural gas is an expensive fuel.
In U.S. Pat. No. 4,960,059 natural gas is injected along with pulverized coal into the primary combustion zone of the furnace to eliminate the need for reburn zone combustion. As an alternative the patent teaches that natural gas be used as the sole injected fuel in the reburn stage of the furnace. Thus this patent also fails to meet the need for implementing reburn combustion in a more economical manner which would use a less expensive fuel.
In U.S. Pat. No. 5,078,064 an apparatus and a process is disclosed wherein pipes, orifices, nozzles, diffusers, ceramic socks and porous ceramic bodies are employed to allow the natural gas reburn fuel to diffuse slowly into the flue gas. Although these techniques work they cannot be precisely controlled. Also since natural gas is used as the sole reburn fuel this patent also fails to provide a more economical reburn technique for NO.sub.x reduction.
In U.S. Pat. No. 5,181,475 there is provided an apparatus and process for the control of nitrogen oxide emissions in combustion products by injecting vortices of a combustible fluid into flue gas. Vortex ring generators introduce natural gas into combustion products in the reburn zone of the furnace as vortices which provide a thorough mix of natural gas and combustion products to eliminate excess air requirements. This process again has the same mentioned failings as all the other previously mentioned devices and techniques.
In view of the foregoing it is seen that an economical apparatus and fuel was needed to accomplish NO.sub.x reduction by reburn zone combustion.